You will need

- - tools for cutting wire;
- - table of specific resistances;
- - additional resistance.

Instruction

1

Identify the substance from which made the guide. Find the specific

**resistance**using the table. Lower**the resistance**of the conductor having the same conductor, only of substances, the specific**resistance**is lower. How many times will smaller this value is the number of times decreases**the resistance**of the conductor.2

If possible, reduce the length of the conduit, which is used in the circuit. Resistance is directly proportional to conductor length. If you shorten the conductor in the n times, then

**the resistance**decreases by the same factor.3

Increase the cross-sectional area of the conductor. Install conduit with a large cross-section or connect several conductors in parallel in the bundle of wires. How many times will increase the cross-sectional area of the conductor, as many times decreases

**the resistance**of the conductor.4

You can combine these methods. For example, to reduce

**the resistance**of the conductor 16 times, replaced it with a conductor, specific**resistance**of less than 2 times reduce 2 times its length and cross-sectional area 4 times.5

To reduce

**resistance**on the part of the chain, attach it in parallel to another**resistance**, the value of which calculate. Note that when parallel connection,**the resistance**of circuit is always less than the smallest resistance in the parallel branches. Calculate the required**resistance**that must be attached in parallel. To do this, measure**the resistance**of the circuit R1. Determine the**resistance**, which should it be – R. then, determine**the resistance**R2, which should be attached to the resistance R1 in parallel. To do this, find the product of the resistances R and R1 and divide by the difference between R1 and R (R2 = R • R1 / (R1 - R)). Note that condition R1 is always larger than R.# Advice 2 : How to find total resistance

**Resistance**is a kind of ability of the circuit to prevent the passage through it of an electric current. They have a variety of materials, such as copper, iron and nichrome. The total resistance is the resistance of the entire electric circuit as a whole. It is measured in ohms. Need to know the resistance of the circuit to estimate short circuit currents and selection of switching devices.

You will need

- The ohmmeter, measuring bridge, calculator.

Instruction

1

To start, determine how the connected elements of an electric circuit relative to each other, as this affects the calculation of the total resistance. The conductors may be in serial or parallel connection. The serial connection is a connection when all elements are linked so that the enclosing the portion of the circuit has a single node, and parallel connection is a connection, when all the circuit elements connected with two nodes and do not have relationships with other nodes.

2

If you have determined that the conductors in an electrical circuit are connected in series, find the total resistance will not be difficult. Simply add up the resistance of all elements. If you are given the resistance of each conductor, but given their voltage and current of any circuit element, then adding all the voltage, you will know the total voltage. The current strength of each element in a series connection are equal, that is, the total current in the entire circuit equal to a current of any conductor of this chain. And then to find the total resistance, divide the total voltage into amperage.

3

If the elements are connected in parallel, the total resistance can be found as follows: multiply the resistance of all conductors and divide by their sum. If you are given the resistance of each element but their amperage and voltage of any circuit element, then adding all the amperage you will learn common. Voltage of each element in parallel connection are equal, that is, the total voltage across the circuit equals the voltage of any conductor to this chain. And then to find the total resistance, divide the voltage to the total current.

4

To determine the total resistance in a circuit, use measuring instruments such as ohmmeter and measuring bridge. They will help you to determine electric resistance.

Useful advice

Be sure to define a method of connecting components in an electrical circuit, as it depends on the correct calculation of the total resistance!

# Advice 3 : How to calculate the wire resistance

**The resistance**

**wire**shows the extent to which it prevents the passage of electric current. Measure it with tester, switched to the operating mode of the ohmmeter. If this is not possible, you can calculate it in different ways.

You will need

- tester;
- - ruler or measuring tape;
- calculator.

Instruction

1

Measure the resistance

**of the wire**. For that its ends connect the tester included in the mode of operation of the ohmmeter. The device screen will be an electric resistance**wire**in ohms or multiples of them are values, depending on instrument settings. The wire must be disconnected from the power source.2

Calculate the resistor using tester, which operates in the mode ammeter and voltmeter. If the wire is a plot of the electric circuit, connect it to the power supply. To the ends

**of the wire**in parallel connect the tester included in the operating mode of the voltmeter. Measure the voltage drop across the wire in volts.3

Switch the tester in the mode of operation of ammeter and switch it into the circuit in series. Get the value of current in circuit in amperes. Using the ratio obtained from Ohm's law, find the electric resistance of the conductor. To do this, divide the value of the voltage U and the current I, R=U/I.

4

Example. The measurement showed that the voltage drop on the conductor 24 V, current is 1.2 A. Determine the resistance. Find the ratio of voltage to current R=24/1,2=20 Ohms.

5

Find the resistance of the

**wire**, not connecting it to the power source. Find out what material made the wire. In the specialized table, find the resistivity of material in Ohm∙mm2/m.6

Calculate the cross-section

**of the wire**, if it is not specified initially. To do this, clean it from isolation if it is isolated, and measure the diameter of conductor in mm. Determine its radius by dividing the diameter by the number 2. Determine the cross section**of the wire**by multiplying the number π≈3,14 the square of the radius of the core.7

With a ruler or tape measure measure the length

**of wire**in meters. Calculate the resistance**of the wire**by multiplying the resistivity of the material ρ on the length of the conduit l. Divide the result by its cross section S, R=ρ∙l/S.8

Example. Find the resistance of copper

**wire**with a diameter of 0.4 mm with a length of 100 m. the resistivity of copper equal to 0,0175 Ohm∙mm2/m. the Radius**of the wire**is equal to 0.4/2=0.2 mm. cross-Section S=3,14∙0,22=0,1256 mm2. Calculate the resistance by the formula R=0,0175∙100/0,1256≈14 Ohms.